Hi,
i have had mine for a while now and this is new to me, on such a high end set this is not right and will need to be addressed, your first port of call is the supplying retailer, however be firm as this group of retailers are not really that hot on the exchange front.
back to the problem in question, the only thing other than earlier un uniformity issues was the energy saving switch which is mounted on the rear lower quadrant of the set, this has been known to "buzz" a little so i would check that area first.
in any event it will need attention and since its still under guarantee that will be the way forward, personally i would not leave it in the hands of Mastercare, i am surprised however that since this has done this from day one you never took it straight back for an immediate replacement.
